1. Field of the Invention
The present invention relates to a system and method for producing multiphase fluid (i.e., oil, gas and water) either downhole or at surface using artificial lift methods such as Electric Submersible Pump (ESP), Wet Gas Compressor (WGC) and Multi-Phase Pump (MPP).
2. Description of the Related Art
Downhole artificial lift or surface pressure boosting are often required to increase hydrocarbon production and recovery. The production fluids are often a mixture of gas, oil and water. In the case of an oil well, the operating pressure downhole can be below the bubble point pressure or the well can have gas produced from the gas cap together with the oil. For gas wells, the gas is often produced with condensate and water.
Electric Submersible Pump (ESP) is an artificial lift method for high volume oil wells. The ESP is a device which has a motor close-coupled to the pump body. The entire assembly is submerged in the fluid to be pumped. The ESP pump is generally a multistage centrifugal pump can be hundreds of stages, each consisting of an impeller and a diffuser. The impeller transfers the shaft's mechanical energy into kinetic energy of the fluids, and the diffuser converts the fluid's kinetic energy into fluid head or pressure. The pump's performance depends on fluid type, density and viscosity. When free gas is produced along with the oil and water, gas as bubbles can build up on the low pressure side of the impeller vanes. The presence of gas reduces the head generated by the pump. In addition, the pump volumetric efficiency is reduced as the gas is filing the impeller vanes. When the amount of free gas exceeds a certain limit, gas lock can occur and the pump will not generate any head/pressure.
To improve ESP performance, a number of techniques have been developed. These solutions can be classified as gas separation/avoidance and gas handling. Separation and avoidance involves separating the free gas and preventing it from entering into the pump. Separation can be done either by gravity in combination with special completion design such as the use of shrouds, or by gas separators installed and attached to the pump suction. The separated gas is typically produced to the surface through the tubing-casing annulus. However, this may not always be a viable option in wells requiring corrosion protection through the use of deep set packers to isolate the annulus from live hydrocarbons. In such environments, the well will need to be completed with a separate conduit for the gas. To utilize the gas lift benefit, the gas can be introduced back to the tubing at some distance from the pump discharge after pressure equalization is reached between the tubing and gas conduit. To shorten the distance, a jet pump can be installed above the ESP to “suck” in the gas. All these options add complexity to well completion and well control.
Gas handling is to change the pump stage design so that higher percentage of free gas can be tolerated. Depending on the impeller vane design, pumps can be divided into the following three types: radial, mixed and axial flow. The geometry of radial flow pump is more likely to trap gas in the stage vanes and it can typically handle gas-volume-fraction (GVF) up to 10%. In mixed flow stages, since the fluid mixture has to go through a more complex flow pass, mixed flow pumps can typically handle up to 25% free gas with some claiming to be able to handle up to 45% free gas. In an axial flow pump, the flow direction is parallel to the shaft of the pump. This geometry reduces the possibility to trap gas in the stages and hence to gas lock. Axial pump stages can handle up to 75% free gas, but have poor efficiency compared to mixed flow stages.
For gas wells, as fields mature and pressure declines, artificial lift will be needed to maintain gas production. Conventional artificial lift with ESP, Progressing Cavity Pump (PCP), and Rod pump all requires separation of gas from liquid. The liquid will be handled by pumps and the gas will flow naturally to surface. Downhole Wet Gas Compressor (WGC) is a new technology that is designed to handle a mixture of gas and liquid. Yet, at the current stage, it still has a limited capability to handle liquid.
At the surface, the conventional approach is to separate the production into gas and liquid and use a pump for the liquid and a compressor for the gas. Two motors are required with this approach, which results in a complex system. Surface MPP and WGC are costly, complex and many times still suffer from reliability issues.
There is presently a need to develop a compact system for downhole artificial lift or surface pressure boosting that works satisfactorily with a wide range of GVF. We have invented a system and method for producing such multiphase fluid downhole and at surface, with resultant overall improved efficiency.